There are many methods of measuring current in a conductor. One approach is to measure the strength of the magnetic field surrounding the wire. This approach involves surrounding the current carrying conductor with a core containing windings to form a transformer. This approach has drawbacks because current probes of this type are bulky, relatively expensive and may not necessarily work at DC.
The measurement of current is particularly useful in switch-mode power supplies where the phase current flows through an inductor where one side of the inductor may be switched. For example, in a buck regulator, one side of an inductor is alternately switched between an input voltage and typically ground causing the rate of change of current flow to switch alternately between positive and negative values. Here, the inductor serves as a kind of flywheel. Measuring the time varying current flow through a phase inductor is useful for many reasons, especially when multiple phases are interleaved. In a multi-phase supply, the analysis of the current through each phase inductor is very useful for the study of current sharing in the supply, but even in single phase supplies, the output current is monitored in current mode feedback scenarios.
In U.S. application Ser. No. 15/803,309, previously incorporated by reference, a method and apparatus is put forth for measuring current through a switching power supply phase by processing the differential voltage measured across an inductor in a phase of the supply. This method is very useful, but other ways may be desirable, especially when trying to correlate measurements.
In some situations, a series sense resistor is available. Sometimes the series sense resistor is already included in a device under test (DUT) and is itself parasitic in nature, like the resistance of a printed circuit board (PCB) trace, a via, or a pin. In other cases, the series sense resistor is introduced through added circuitry used to capture test measurements from the DUT. Either way, this is a resistance, typically small valued, in series with a circuit path where current measurement is wanted. The resistor is typically as small as possible because any current delivered by the path causes power to be dissipated in the resistor according the well known equation: P=I2R. Furthermore, there is a voltage drop across the resistor of: V=I·R. Generally, a small resistance value is preferred to minimize the effects of the measurement circuitry on the output. However, in current sense applications, the voltage drop must be large enough to be measured by a differential voltage probe. The differential voltage balancing act typically involves a sense resistor value on the order of a milliohm (mΩ) for a current measurement in a supply on the order of ten amps.
With such a small resistance value, a problem that arises is that of parasitics. A parasitic is an unwanted and usually unremovable extra circuit element in the model of the element. A wire has parasitics in the form of some resistance, and also generates an electric field between other wires and ground causing the wire to have some capacitance. Furthermore, current flowing through a wire causes a magnetic field to develop that leads to inductance. At low frequencies and for large values of resistance for a resistor component, the parasitic resistance, capacitance and inductance can often be neglected, but for higher frequencies and small values of resistance, the parasitics can cause very poor measurements of current when differential voltage is used to infer current flow. Inaccurate measurements may have significant implications for the design, analysis, validation, and debugging of various electronic circuits. In the context of switch-mode power supplies (SMPS), for example, faulty measurements may lead to poor design choices, potentially compromising the stability and power integrity of the device.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.